Method and device for launching aerial vehicles

ABSTRACT

A heavier-than-air air vehicle, particularly a long endurance, solar powered, unmanned aerial vehicle (UAV) intended for “perpetual” flight within the stratosphere, is carried to its operational altitude suspended on a tether from a helium balloon. The tether is attached at or towards a tip of the UAV&#39;s wing so that it is carried in effectively a 90° banked attitude. At the desired altitude the UAV&#39;s powerplant is started and it flies on its tether in an upwardly-spiralling path relative to the balloon until a level or near level attitude is attained, when the tether is released and the UAV is permitted to assume free flight.

The present invention relates to the launching of aerial vehicles and ismore particularly (though not exclusively) concerned with the launchingof high altitude, long endurance, solar powered, winged, unmanned aerialvehicles (UAVs).

Solar powered UAVs have been proposed for use as long endurance aerialplatforms, flying at stratospheric altitudes (typically between 15,000and 30,000 m), for such roles as communications relay, earthobservation, weather monitoring, surveillance and mapping. In thisrespect they may provide a cost-effective alternative to theearth-orbital satellites conventionally used for these purposes and havethe additional advantage that they can be easily controlled to remain insight of a selected part of the globe or to fly to any other selectedstation. Such vehicles comprise a lightweight structure including atleast one aerodynamic lifting surface (wing), one or moreelectrically-driven thrust-producing motors, an array of photovoltaiccells, and electric storage means such as rechargeable batteries orregenerative fuel cells. An example of such a vehicle is described inU.S. Pat. No. 5,810,284, WO 01/58756, WO 01/58757 and WO 01/58758. Inprinciple vehicles of this kind can remain aloft indefinitely, limitedonly by the reliability of their mechanical components and airframe.integrity, by following a diurnal flight pattern in which, during thehours of daylight, the power generated by the photovoltaic cells is usedto drive the motors at high power to climb the vehicle from a baseoperational altitude to a higher operational altitude and to charge theelectric storage means, and during the hours of darkness the motors aredriven at lower power from the storage means and the vehicle is allowedto descend to the base (though still stratospheric) operationalaltitude.

Atmospheric conditions within the stratosphere are generally benign,meaning that an airframe designed to fly only within the stratospherecan be less robust, and hence lighter, than one designed to fly withinthe variable atmospheric conditions of the troposphere. Furthermore theminimisation of airframe weight is of especial importance to the successof a long endurance vehicle whose power supply is limited to theelectricity which can be generated from onboard photovoltaic callsduring daylight.

The conventional method by which a high altitude, solar powered UAV isinitially launched to its operational altitude—such as the one describedin U.S. Pat. No. 5,810,284 et al —is for it to climb under its own powerfrom the ground. This implies, however, that the airframe must besufficiently strong (and therefore heavy) to cope with the additionalloads imposed by flight through turbulent regions of the troposphere(hence imposing an undesirable limitation on the payload which it cancarry) and/or that it will risk airframe damage during the climb and/orthat its launch must await the most favourable weather conditions.

U.S. Pat. No. 4,697,761 discloses an alternative method of launching asolar powered UAV in which it is carried aloft, suspended in a 90°banked attitude, inside the envelope of a lighter-than-air balloon. Whena desired altitude is reached the lower end of the balloon is unreefedand, after a brief delay, the UAV is released and free falls from theballoon before assuming level flight under its own power. No details areprovided of aerodynamic control required to transition from the freefall 90° banked attitude to level flight. It is likely, however, thatsignificant loads would be imposed upon the airframe in recovering fromthe drop from the balloon, meaning again that it must be stronger (andtherefore heavier) than that required for normal stratospheric flight.

It is one aim of the invention to avoid the above-described drawbacks ofprior art launching methods and accordingly in one aspect the inventionresides in a method of launching a heavier-than-air air vehiclecomprising at least an aerodynamic lifting surface and thrust-producingpowerplant, the method comprising: raising the heavier-than-air vehicleto a desired altitude by means of a lighter-than-air vehicle; when saiddesired altitude has been reached, operating said powerplant to producethrust with the heavier-than-air vehicle suspended from thelighter-than-air vehicle by means of a tether attached to theheavier-then-air vehicle at or towards an extremity of said liftingsurface, whereby to cause the tethered heavier-than-air vehicle todescribe an upwardly-spiralling path relative to the lighter-than-airvehicle; and releasing said tether to permit free flight of theheavier-than-air vehicle when the latter has reached a desired attitude.

The method according to the invention is particularly applicable to thelaunching to stratospheric altitude of a long endurance, solar poweredUAV and obviates the need to design the airframe of the latter forflight under its own power through the troposphere or to withstand afree fall, unusual attitude release from the lighter-than-air vehicle.

The invention also resides in the combination of a heavier-than-air airvehicle comprising at least an aerodynamic lifting surface andthrust-producing powerplant; a lighter-than-air vehicle adapted to liftthe heavier-than-air vehicle; a tether attachable to theheavier-than-air vehicle at or towards an extremity of said liftingsurface, for suspending the heavier-than-air vehicle from thelighter-than-air vehicle; and control means; all constructed andarranged so as to execute the foregoing method.

The invention also resides in a solar powered, unmanned heavier-than-airair vehicle adapted to be launched by a method as defined above, saidvehicle comprising at least an aerodynamic lifting surface and athrust-producing powerplant, the structure of said lifting surfaceincorporating means at or towards an extremity thereof for attachment toa tether for launch of the vehicle as aforesaid.

These and other aspects of the present invention will now be moreparticularly described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a pictorial view of one embodiment of a high altitude, longendurance, solar powered UAV which may be launched by a method accordingto the invention; and

FIGS. 2 to 5 are schematic diagrams, not to scale, illustratingsuccessive stages in the launching to high altitude of the UAV of FIG.1.

Referring to FIG. 1, the illustrated UAV 1 comprises a tubular fuselage2 with a mainplane 3 and an inverted V tail 4, (although in otherembodiments a more conventional empennage may be employed). Themainplane 3 has dihedral tip sections 5 and substantially the whole ofits upper surface carries arrays of photovoltaic cells 6, or such cellsmay be housed within the mainplane structure beneath a transparent upperskin. Its powerplant comprises a pair of wing-mounted brushless DCelectric motors (not seen) driving propellers 7, although otherembodiments may comprise a different number of such powerplant dependingon the size of airframe and motor rating. Housed within the mainplanestructure are a plurality of regenerative fuel cells or rechargeablebatteries.

In use of the UAV, having been launched to a desired stratosphericaltitude, power is supplied by the photovoltaic cell arrays 6 to itsmotors and to charge the onboard fuel cells or batteries during thehours of daylight, and power stored by the fuel cells/batteries issupplied to the motors during the hours of darkness. In this way thevehicle is capable of “perpetual” flight in accordance with knownprinciples. The airframe weight is minimised, the fuselage 2 and wingand tail spars and ribs being constructed of carbon composite, leadingedge mouldings of a high performance rigid foam material such asRohacell®, and the wing and tail surfaces a Mylar® skin. Roll control isprovided by ailerons 8 and the tail 4 is all-moving to provide pitch andyaw control; that is to say each plane of the tail 4 is rotatablymounted to the fuselage 2 by a respective spar and they can be turned inunison to function as an elevator (pitch control) and differentially tofunction as a rudder (yaw control).

Control of the vehicle's flying controls, powerplant, payload equipmentand the tether release to be described below can be exercised via aradio link from the ground or via satellite to a remote ground stationand/or by an onboard computer. Its payload can be carried in the regionof the fuselage/wing root junction or distributed in multiple modulesacross the wing 3 and will comprise such communications, monitoring orother equipment as may be appropriate for its operational role, examplesof which are mentioned in the introduction.

Turning now to FIGS. 2 to 5 these illustrate successive stages in thelaunch procedure for the vehicle.

With reference to FIG. 2, the UAV 1 is transported to the launch siteclamped horizontally onto a spanwise beam 9 within a container 10. Thebeam 9 is pivoted to the container 10 at one end as indicated at 11. Atthe launch site the top 12 of the container is removed and its end 13adjacent to the pivot 11 is opened. A tether 14 from a helium balloonenvelope is attached first to the end 15 of the beam 9 opposite to thepivot 11 and thence to the spar of the mainplane 3 of the UAV at theadjacent outermost end of its centre section, before the tip section 5.The balloon is inflated and rises as indicated at 16 in the Figure. Theballoon may be of a generally conventional type as used for example inweather monitoring or scientific ballooning and has a design floataltitude with payload (in this case the UAV 1) in the region of20,000-30,000 m. It is equipped with a vent valve and a quantity ofballast (not shown) the release of which can be controlled via a radiolink and/or by an onboard computer. A mass 17 is included in the tether14 for a reason which will be described later. It is noted that for easeof illustration the balloon 16 is shown much reduced in scale from itsactual relative size to the UAV 1 and the tether 14 is likewise shownmuch shorter. In practice the tether may be in the region of 200-400 min length. Similarly, the displaced angle of the tether 14 under theinfluence of wind on the balloon 16 is exaggerated in FIGS. 2 and 3 fromthat which is likely to obtain in practice.

The lift of the balloon 16 on the beam 9, aided by jacks 18 beneath thebeam, causes the beam to turn about its pivot until it reaches thevertical condition illustrated in FIG. 3. In this condition the clampsfrom the beam to the UAV 1 are released and the UAV is suspended on thetether 14 from the end 15 of the beam effectively in a 90° bankedattitude. The tether 14 is then released from the beam 9 and the balloon16 lifts the UAV up and away from the beam. The container 10 will havebeen orientated originally with the beam end 15 into wind (the directionof which is indicated by the arrow W in FIG. 3) so there is no risk ofthe UAV colliding with the beam during this part of the launchprocedure.

The balloon 16 ascends to a desired stratospheric altitude carrying theUAV 1 beneath it on the tether 14 in the same attitude as that fromwhich it was released from the beam 9, as indicated in FIG. 4. At thetop of the ascent, power is switched to the UAV's motors and its tail 4is placed in a “down elevator” position. The combined effect is that theUAV commences to describe an arcuate path (conical pendulum motion)about the point of connection of the tether 14 to the mass 17, with theupper surface of the wing 3 facing outwards. When a stable motion hasbeen achieved, the elevator is set to neutral and the power isincreased. As the speed of the UAV increases, the radius of its pathincreases, and the lift which it generates increases, so that thetethered UAV describes an upwardly-spiralling path relative to theballoon, with its bank angle correspondingly reducing. FIG. 5 indicatesthe disposition of the two vehicles at one point during the upwardflight of the UAV 1 relative to the balloon 16 when the UAV has reachedan angle of approximately 45° to the horizontal. The process continuesuntil the UAV has been brought up to a horizontal attitude, or nearlyso, when it is released from the tether 14 and permitted to assume freeflight.

The purpose of the mass 17 within the tether 14 is to define arelatively stable anchor point about which the UAV I can fly in thecourse of its transition from the FIG. 4 condition to the horizontal ornear horizontal attitude in which it is ready to be released from thetether. Nevertheless it will be appreciated that the tendency of the UAVflying around the balloon on its tether will be to pull the balloonaround in an arcuate path, albeit of much reduced radius compared to thepath of the UAV and lagging behind it by several degrees. The tendencyof this pull on the UAV itself is to yaw it inwards towards the centreof its arc, although this tendency can be corrected by positioning thepoint of attachment of the tether 14 to the UAV to lie slightly behindthe centre of gravity of the vehicle and/or by the application ofopposite “rudder” (differential tail plane) control. It will also beappreciated that as the UAV develops increasing amounts of lift in itsflight around the balloon and supports an increasing proportion of itsown weight the effect is to relieve the weight on the balloon and thetendency would therefore be for the balloon to rise. If permitted to doso it could prevent the UAV from reaching the horizontal or nearhorizontal attitude in which it is ready to be released so the tendencyis corrected by the controlled venting of gas from the balloon.

The attitude of the UAV is monitored throughout this phase of the launchprocedure and its power, flying controls and the balloon buoyancy arecontrolled as required to ensure that it reaches the requisiteconditions of airspeed and attitude for release from the tether. To thisend both the UAV 1 and the balloon 16 may be equipped with GPS receiverswith telemetry links to the ground and the attitude of the UAVdetermined from the relative altitudes of the two vehicles, and/or theballoon may carry a video camera through which the UAV can be observed.

The main length of the tether 14 may be made of any suitable materialsuch as Kevlar® or Spectra® but to effect its release when required itmay incorporate at least a link of polyethylene or other fusible rope(e.g. Spectra®) at the attachment to the UAV, around which a heatingelement is wound. When the signal is given to release, the heatingelement is energised and melts the link.

After the UAV 1 has been released, the balloon 16 may be fully ventedand safely returned to the ground for recovery by an onboard parachute.If eventually the UAV is to be recovered, it may be “dethermalised”(wing 3 fully stalled) and flown to the ground with directional controlprovided by differential operation of the tail planes. This reducesflight loads during descent, provides a more rapid controlled descent ata slower airspeed than conventional gliding flight and avoids the weightpenalty of the UAV carrying a recovery parachute.

It will be appreciated from the foregoing description that the procedurefor launching the UAV 1 to its operational stratospheric altitude doesnot require that it is flown up through the troposphere under its ownpower but rather is supported in its passage through the troposphere bythe balloon 16. On the other hand the transition to free flight once theballoon has carried it to altitude is accomplished under its own powerin a gradual and controlled manner without any free-fall drop from theballoon. It follows that the airframe can be lighter than one which isdesigned to cater for the aerodynamic loads imposed by troposphericflight or a recovery from a free fall, with a corresponding increase inthe payload which can be carried.

Although it will usually be the case that the UAV is raised by theballoon suspended in the attitude indicated in FIG. 4, it is within thescope of the invention for it to be raised in a different attitude andtransitioned to the FIG. 4 attitude for the commencement of its poweredphase. It is also within the scope of the invention for the UAV to beraised on a shorter tether than that required for its transition to freeflight and for the tether to be payed out as required for thecommencement of its powered phase.

1. A method of launching a heavier-than-air air vehicle comprising atleast an aerodynamic lifting surface and thrust-producing powerplant,the method comprising: raising the heavier-than-air vehicle to a desiredaltitude by means of a lighter-than-air vehicle; when said desiredaltitude has been reached, operating said powerplant to produce thrustwith the heavier-than-air vehicle suspended from the lighter-than-airvehicle by means of a tether attached to the heavier-than-air vehicle ator towards an extremity of said lifting surface, whereby to cause thetethered heavier-than-air vehicle to describe an upwardly-spirallingpath relative to the lighter-than-air vehicle; and releasing said tetherto permit free flight of the heavier-than-air vehicle when the latterhas reached a desired attitude.
 2. A method according to claim 1 whereinthe heavier-than-air vehicle is raised by the lighter-than-air vehiclesuspended as aforesaid.
 3. A method according to claim 1 wherein theheavier-than-air vehicle is initially supported by a pivotable structurein a substantially level attitude; said structure is pivoted to placethe heavier-than-air vehicle in a substantially 90° banked attitude; andthe heavier-than-air vehicle is released from said structure whiletethered to the lighter-than-air vehicle to be raised as aforesaid.
 4. Amethod according to claim 3 wherein the lighter-than-air vehicle isattached to said structure to cause or assist the same to be pivoted asaforesaid.
 5. A method according to claim 1 wherein the heavier-than-airvehicle has pitch control means which are caused to be in a pitch downcondition at the commencement of said operation of said powerplant.
 6. Amethod according to claim 1 wherein the heavier-than-air vehicle has yawcontrol means which are caused to be in a condition tending to yaw thevehicle opposite to its attachment to said tether during the course ofit describing said upwardly-spiralling path.
 7. A method according toclaim 1 wherein the position of attachment of said tether to theheavier-than-air vehicle is aft of the centre of gravity of suchvehicle.
 8. A method according to claim 1 wherein gas is caused to bevented from the lighter-than-air vehicle during the course of theheavier-than-air vehicle describing said upwardly-spiralling path.
 9. Amethod according to claim 1 wherein a mass is incorporated within saidtether to define a position about which the heavier-than-air vehicledescribes said upwardly-spiralling path.
 10. A method according to claim1 wherein said tether includes a length of fusible material adjacent toits attachment to the heavier-than-air vehicle and the heavier-than-airvehicle includes heating means for melting the same to release thetether.
 11. A method according to claim 1 wherein said desired altitudeis within the stratosphere.
 12. A method according to claim 1 whereinsaid desired attitude is level, or nearly so.
 13. A method according toclaim 1 wherein said heavier-than-air vehicle is a solar powered,unmanned vehicle.
 14. In combination: a heavier-than-air air vehiclecomprising at least an aerodynamic lifting surface and thrust-producingpowerplant; a lighter-than-air vehicle adapted to lift theheavier-than-air vehicle; a tether attachable to the heavier-than-airvehicle at or towards an extremity of said lifting surface, forsuspending the heavier-than-air vehicle from the lighter-than-airvehicle; and control means; all constructed and arranged so as toexecute a method according to any preceding claim.
 15. The combinationof claim 14 also comprising a structure adapted to support theheavier-than-air vehicle on the ground in a substantially level attitudeand being pivotable to place that vehicle in a substantially 90° bankedattitude at the commencement of its launch.
 16. A solar powered,unmanned heavier-than-air air vehicle adapted to be launched by a methodaccording to claims 1, said vehicle comprising at least an aerodynamiclifting surface and thrust-producing powerplant, the structure of saidlifting surface incorporating means at or towards an extremity thereoffor attachment to a tether by which the vehicle can be suspended forlaunch of the vehicle as aforesaid.
 17. A vehicle according to claim 16incorporating means for the selective release of said tether.
 18. Avehicle according to claim 16 wherein said lifting surface comprises acentre section and respective dihedral tip sections and said means ofattachment are disposed adjacent to the junction of said centre sectionand a said tip section.
 19. A vehicle according to of claims 16 incombination with a structure adapted to support the vehicle in asubstantially level attitude and being pivotable to place the vehicle ina substantially 90° banked attitude at the commencement of its launch.